DHCP Server and IP Helper

[Andre Sydoriak]

Hi all,

if we have two VLANs (red and green) configured on a Cisco 3750X is it possible to let DHCP requests from red to be forwarded to a central DHCP server using ip helper and clients on the green VLAN get their IP address configured from DHCP server sitting on Cisco 3750X?

At the moment it seems to be that if ip helper is configured for red, dhcp server of green isn't working and clients do not get any IP address on green while red is working like a charm.

Thanks for help,

André

[Robert Falconer]

Which vlan is the DHCP server on, red or green?

[Andre Sydoriak]

It is a windows dhcp server which will be used from red using ip helper. Windows server sits on different vlan say yellow. this is working.

But I have configured service dhcp on 3750x with a scope for green (172.16.17.0/24) which is not working at the moment.

Is it possible to use both ip-helper and service dhcp on one 3750x or either ip-helper or service dhcp is working on one 3750x`?

[paul driver]

Hello

"It is a windows dhcp server which will be used from red using ip helper. Windows server sits on different vlan say yellow. this is working."

In that case you need to forward the dhcp request towards the server and to that means relay is required

For the wintel dhcp server I assume a scope for each vlan is created and authorised ( if in active directory) and enabled
From the switches perspective apply the IP helper address of the dhcp server on each SVI of the scope vlans

If you are using secondary addressing on a single SVI then you will need to apply -Ip dhcp smart-relay to this SVi.

Res
Paul

Sent from Cisco Technical Support iPad App

[Robert Falconer]

Ip helper is required on the SVI for red vlan.

As Paul says, you will need to have a scope created on your DHCP server that is the IP range of the red vlan.

You can run a dhcp server on the 3750 for the green vlan if you want. It would be easier to just use a helper on that vlan as well and forward it to your DHCP server. Again, you would need to create a scope for the green vlan as well.

[mvsheik123]

Hi Andres,

It sounds like your DHCP server is in vlan 'red'. You need to configure 'ip helper-address on 'green' vlan interface. Make sure you have DHCP scope configured and active on DHCP server for 'green' vlans ip range.

hth

MS

[cadet alain]

Hi,

Could you post your config.

Does green scope work without the ip helper on RED SVI ?

Regards

Alain

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[John Blakley]

"if we have two VLANs (red and green) configured on a Cisco 3750X is it  possible to let DHCP requests from red to be forwarded to a central DHCP  server using ip helper and clients on the green VLAN get their IP  address configured from DHCP server sitting on Cisco 3750X?"

The dhcp scope on the switch should match the green vlans subnet if you're wanting green to get an address from the switch. If you have the dhcp server on the yellow vlan, your red vlan svi needs to have the helper pointing to the server in the yellow vlan to get addresses for the red vlan. (First time I've worked with colors as vlans, so bear with me.)

In other words, if your green svi address is 172.20.20.1/24, your dhcp scope on the switch should support 172.20.20.0/24. Your red vlan can support 192.168.10.0/24 and have a scope on the yellow dhcp server for 192.168.10.0/24.

HTH,
John

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[Andre Sydoriak]

Dear all,

thanks for help. Using ip helper and creating the scope on the server worked. :-)

Another question:

I created a VLAN 500 (172.16.17.0/24) which will be used for guests joining the guest WiFi. At the moment we have one Aruba IAP93 Access Point for WiFi. From the switch to the AP we have a trunk port and the VLAN is distributed via VTP. When I connect a Laptop to the WiFi it get's an IP address from the VLAN 500 scope but we aren't able to ping the gateway which is 172.16.17.1/24. When I connect the laptop to the VLAN using network cable it works.

It seems to be that VLAN 500 interface is down till some active devices connect to that VLAN and comes up. What can I do?

Thanks,

André

[devils_advocate]

Hi André

Its normal for a Vlan interface to be down until a device in that Vlan comes online but if you have a Trunk link that 'permits' that Vlan, the interface should come up.

Is the Vlan interface on the same switch the AP plugs into?

Can you post the config for the Trunk port to the AP?

Thanks

[Andre Sydoriak]

Hi,

ok.

The AP is plugged into another switch not on the switch where the Vlan interface is configured.

Here is the config of the trunk port connecting the AP (smartport cisco-wirless applied)

interface GigabitEthernet1/0/1

description Uplink AP1

switchport trunk native vlan 200

switchport mode trunk

switchport nonegotiate

srr-queue bandwidth share 1 30 35 5

priority-queue out

mls qos trust cos

macro description cisco-wireless

auto qos trust

spanning-tree bpduguard enable

!

[devils_advocate]

Hi Andre

The config looks fine.

To clarify, when the AP is plugged in, the Vlan interface shows as Down but if you plug a PC into that same port, the Vlan interface comes up?

How is that switch (the one the AP is patched into) connected to the switch that the Vlan interface is configured on? Trunk port?

Thanks

[Andre Sydoriak]

Hi,

it seems to be that the switch had problems with that particular subnet. I changed it to a standard private class C and suddenly it worked like a charm. Don't know why, but it works.

Thanks for your help,

André